I'm using Linux, but one of my solution should work for you. First, get yourself a serial terminal emulator (I don't know what the current ones are for 64-bit Windows). Set the comms parameters to 9600 bps, 8bit, no parity. You can then talk to the Psion via the Psion Comms application (remote link switched OFF on the Psion). Psion Comms supports Y-modem protocol, so while you need to transfer the files directly into the right folders on the Psion, you can do it a directory at a time.
The second option involves setting up DOSBox if you haven't already. Get it talking to your serial port, preferably as COM1 within DOSBox. Get a copy of the MCLINK software for DOS. Connect the Psion to the PC with remote link ON, and load MCLINK within DOSBox. You should then be able to transfer files to and fro using the commands in MCLINK (it has a TXT file) or using the Psion user interface.
There's a helpful and friendly Psion Discord if you get stuck, you can find the link on the Psion community web site https://psion.community/